Modern wind turbines are of such a size that the wind most rarely is uniform over the entire rotor area. These differences in wind speed, which are a result of turbulence, wind shear, yaw error operation, tower shadow, wake effects etc., results in varying aerodynamic forces on the blades during rotation, causing major fatigue loads on the turbine and in some cases also extreme loads.
Traditional control of pitch regulated turbines are methods of controlling the collective pitch (i.e. all the blades are turned simultaneous to the same angle) settings of all blades to provide an optimised yet well defined power output of the turbine. From U.S. Pat. No. 4,339,666 it is known to control the pitch angles in such a way that maximum loads on the rotor and generator is below maximum allowable values. This patent describes the use of average wind velocity and turbulence as the primary controlling parameters. The disadvantage is however that variation in wind speed over the rotor area is not taken into consideration.
From patent WO0133075 a method of how to control pitch angles of the wind turbine based on measurements of mechanical loads on the turbine in such a way that non-uniform wind load are compensated by individual pitch. The disadvantage is however that due to the large aerodynamic damping of the blades, the mechanical loads will be measured some time after the wind disturbance has occurred.
From patent U.S. Pat. No. 6,361,275 a wind turbine is patented that uses individual pitch for active load reduction. This patent focus mainly on the effects on measuring loads with strain gauges or similar on the blades and the hub. In connection to the load measurements, measurements of the local wind velocity and wind direction on the blades are mentioned. The aerodynamic measurements are a supplement to the load measurements. It is however not stated how to use these different flow sensors, since the focus is on the load measuring properties. The aerodynamic sensors are placed on the trailing edge of the blade and the only way to change the aerodynamic loading is by pitching the blades or parts of the blades.